When the game is about positioning and being able to take/control a position on the stage, and by extension of this it favors consistenty fast movement, Like dash dancing, over short bursts of movement with set up, like perfect wavelands, slower characters are going to have a rough time in a lot of MUs. Zard (and DK, imo) is really fast but doesn't have a lot of tools to create space for himself, but Samus/ROB do, and so do DDD and Yoshi to a lesser extent. I think the problem is that when you have all these faster characters that take/control space better than you and kill just as easily or easier, slow/heavies fall behind.
Basically what I think it boils down to is that I'd much rather be fighting out of the corner vs say, shiek or diddy playing as Fox than if I was playing as Bowser or ganon. Slower=less options to take space quickly=easier to cover all the options and keep them on the back foot. Also generally since the fatties are slower, it means less threatening follow ups if they do manage to fight out. Unless you mess up royalty and get grabbed/side-b'd by the ganon fighting from ledge, you eating what, a fair/Bair and losing stage control (% dependent). Slip up vs a fox or a falcon fighting from the corner and get converted into a shine/grab from the stray convertible hits they throw out to control space.
I don't think slower characters have a place being good in a movement/positional based game, unless they have something ridiculous to cover up weak spots (stuff like 3.02 DK's invincible arms on nair comes to mind.)